a) Field of the Invention
The present invention relates to a ranging method for measuring the flight distance of a flight object such as a rocket and, more particularly, to a ranging method including the steps of inserting ranging signals into a telemeter signal to be transmitted from a telemeter transmitting apparatus mounted on a flight object, transmitting the telemeter signal which contains the ranging signals, and measuring the flight distance of the flight object. The invention also relates to apparatus for accomplishing the method.
b) Description of the Related Art
Several methods have been utilized for measuring the distance to the flight point (i.e., the flight distance) of a flight object such as a rocket. A first method includes the steps of transmitting a ranging signal toward a flight object from an apparatus provided on the ground, measuring the time taken for the ranging signal to travel to the flight object and return to the apparatus provided on the ground, and computing the flight distance from the measured time. A second method (Doppler integration method) includes the steps of receiving at an apparatus on the ground a ranging signal (alternating-current signal) transmitted from a flight object, measuring and integrating the Doppler shift of the received signal, and computing the flight distance from the resultant integral. A third method (asynchronous tone method) includes the steps of measuring the propagation delay phase of the aforesaid received signal and computing the flight distance from the amount of such phase delay. The first method is called a two-way method, and the second and third methods are called one-way methods.
The advantage of the two-way ranging method is that it is possible to measure the flight distance at an arbitrary point in time. However, the two-way ranging method has a number of disadvantages. For example, it is necessary to prepare a flight-object tracking system for ranging purposes separately from a telemeter system, it is necessary to mount on a flight object a transponder, a reflector or a similar device for returning a ranging signal, and it is also necessary to provide both a receiver and a transmitter on a ground side for ranging purposes. These necessities lead to a number of disadvantages such as the need for an extremely complex and expensive system construction.
The one-way ranging method has the advantage of implementing an inexpensive system construction since it is possible to adopt a ranging system including a ground-side part which can be constructed of receiving elements only and hence does not need a large-scale tracking system, and, furthermore, it is not necessary to mount a ranging-signal returning device such as a transponder or a reflector on a flight object (transmitting elements need only to be mounted thereon). However, the one-way ranging method has the following disadvantages. For example, it is necessary to transmit separately a ranging signal and a telemeter signal. In addition, since the ranging signal from a flight object must be received and observed continuously from the time of the start of flight of the flight object, if the reception of the ranging signal from the flight object is interrupted due to any cause, ranging subsequent to the time of occurrence of the interruption becomes impossible.
In other words, in the case of the Doppler integration method, no integral is obtained during the period in which it is impossible to receive the ranging signal. As a result, it becomes impossible to detect any flight distance during such a period and overall ranging computations will be impossible.
In the asynchronous tone method, if the flight distance becomes equal to or greater than the wavelength of a ranging signal (so-called over wavelength), the flight distance is obtained from delay phase within one wavelength and the number of over wavelengths. Accordingly, if any over wavelength occurs during the period in which it is impossible to receive the ranging signal, it is impossible to detect the fact that the over wavelength has occurred and, if so, the number of over wavelengths, so that ranging computations will be impossible.
To increase a distance which can be measured within one wavelength, a signal of long wavelength may be employed. Since this method is disadvantageous in terms of ranging accuracy, the asynchronous tone method uses two or more kinds of signals, such as a signal of long wavelength and a signal of short wavelength, to compute the flight distance on the basis of the amount of phase delay of each of the signals. However, this method needs two or more kinds of signals for ranging purposes and, in addition, if the flight distance exceeds the wavelength of the long-wavelength signal, a problem similar to the aforesaid one takes place. For this reason, the above-described method is not a perfect solution.